The invention concerns a magnet system in a magnetic resonance apparatus for generating a magnetic field which is homogeneous and temporally highly stable within a volume under investigation and extends along a z-axis, with a superconducting magnet coil which is disposed in a cryostat and with a periodically operated refrigerator having magnetic regenerator material which is disposed in a regenerator housing within the cryostat in the stray field of the magnet coil, and which is provided with a device for shielding or compensating a magnetic disturbing field in the volume under investigation generated by periodic operation of the refrigerator through changes in the regenerator magnetization.
U.S. Pat. No. 4,535,595 discloses detection of the magnetic disturbances of a motor-driven piston refrigerator using a sensor means, wherein the sensor means controls a coil for compensation of the disturbing field acting in the sample volume. A detector coil disposed in the region of the refrigerator or in the sample volume serves as sensor means. The signals are guided via an electronic control device to a compensation coil surrounding the sample volume. An arrangement of this type does not, however, provide optimum compensation of the disturbances. Another disadvantage is the need for additional electronic devices.
In contrast thereto, pulse tube coolers effect expansion and compression of a working gas by a shock wave front in a pulse tube.
In addition to piston refrigerators, pulse tube coolers have been used to an increasing extent as refrigerators, wherein expansion or compression of the working gas is effected using a shock wave front. After compression, the working gas flows through a regenerator and is relaxed in an expansion chamber with associated heat exchange between the working gas and the regenerator material. Regenerator materials with phase transitions are generally used to effect this heat exchange at minimum temperatures. The phase transition increases the specific heat of the regenerator material and permits cooling of the working gas to less than 3 K. The materials which are conventionally used for this purpose have a magnetic phase transition which can, however, produce considerable disturbances in the main magnetic field in the volume under investigation.
U.S. Pat. No. 6,029,458 discloses a magnet arrangement with a cryocooler which comprises regenerator materials having a magnetic phase transition. The disturbing fields generated by the magnetic phase transition of the regenerator material are shielded by a superconducting sleeve which surrounds the refrigerator and which is disposed between the cold end of the refrigerator and the volume under investigation. The superconducting sleeve surrounds 90° to 270 ° of the cold end of the refrigerator and contains one or more layers of superconducting material, preferably NbTi. The magnetic disturbing field of the regenerator material induces a magnetic field in the sleeve which is opposite to the disturbing field and keeps the magnetic flux through the sleeve constant, thereby compensating for the disturbing field. The use of such superconducting sleeves is problematic, since they also interact with the magnetic field of the main magnet. The superconducting material which is used is layered and has transverse dimensions of some centimeters. These layers tend to be instable due to the disturbing field of the regenerator material or the main field magnet, and quenches can occur. For this reason, reliable and stable shielding is not possible in practice. The current distributions in superconducting materials tend to be theoretically instable for transverse dimensions of the superconducting materials of more than approximately 0.1 mm. Instabilities are also expected for layered superconducting materials with transverse dimensions of more than approximately 10 mm.
It is the object of the invention to propose a stable shielding device for the above-described magnet system which compensates for the disturbing fields caused by a magnetic regenerator material at the location of the volume under investigation in a simple fashion.